Painted metal containers (e.g., steel or aluminum cans) are now used prolifically for packaging foods and beverages. To that end, in 1999, one corporation manufactured over 37 billion aluminum cans for the United States beverage industry alone. Metal containers such as aluminum cans may be painted, e.g., by rolling individual cans against paint-covered rubber cylinders. Often a clear protective coating is applied to protect this exterior paint coat from smearing, marring, or degradation. Cans are then conveyed through ovens for curing and drying of the coating.
Investigators continue to search for new coating compositions that can be used in container mass production applications. One goal is to develop coating compositions that are ready for immediate use without additional preparation steps (i.e., that are “one-stage”). The compositions of interest must be stable in bulk quantities at standard temperature and pressure so that they may be efficiently and cost effectively shipped and stored without gellation or degradation prior to use. They also must be able to withstand the conditions of processing. When applied to substrates, the compositions must cure and dry rapidly (i.e., possess “snap cure” capabilities), have good adhesion to metal and other materials, be scratch resistant, and have a clear and glossy appearance.
New exterior coating compositions that meet these requirements are being developed. Many of these coatings, however, are variants of formulations employed traditionally in the packaging industry, including phenol/formaldehyde, urea/formaldehyde, and melamine/formaldehyde formulations. However, the widespread use of formaldehyde in production scale applications is falling into disfavor because of perceived environmental and health considerations. Formaldehyde is an eye and skin irritant that is a suspected carcinogen. The unfavorable environmental and toxicological profiles of formaldehyde have spurred researchers to develop new coating formulations that contain formaldehyde scavengers. Unfortunately, such coatings have many shortcomings. For example, resins containing formaldehyde scavengers such as melamine, urea, or ammonia, frequently have low water tolerance and require the need for stabilizing emulsifiers. Rogue formaldehyde emissions, as well as additional, stability-related problems associated with heating and curing operations, also pose complications.
In addition to the environmental concerns about formaldehyde, there is additional interest in reducing the levels of volatile organic compounds (VOCs) in industrial products such as coating compositions. This interest is in part due to governmental regulations limiting the industrial use of and the resulting emissions of these into the atmosphere. Consequently, many of the coating compositions presently available do not meet the new regulations.
As a result, there is a need in the packaging industry for substantially formaldehyde-free hardenable compositions that are easy to use. There is also a need for compositions that exhibit favorable crosslinking and coating characteristics. In particular, there is a need to develop coating compositions and formulations that are stable at standard temperature and pressure (i.e., 25° C. and 1 atmosphere) so that they can be transported and stored in bulk prior to use in production scale operations. There is also a need for coating compositions and formulations that have stability and curing profiles that comport with production scale applications and are snap-curable, that adhere to substrates such as metal, that have a clear, glossy appearance, and that are mar resistant.
It also would be useful to prepare formaldehyde-free roll coating compositions that are non-misting. The coating compositions of the present invention solve these and other problems.